1. Field of the Invention
The present invention relates to a projection exposure apparatus designed to move a photosensitive substrate by a step-and-repeat scheme when a pattern of a mask (reticle) is exposed onto the photosensitive substrate through a projection optical system and, more particularly, to an apparatus designed to optimize a focusing operation with respect to a shot area, on a photosensitive substrate, which is to be exposed.
2. Related Background Art
In a projection optical apparatus of a step-and-repeat scheme (stepper), a wafer mounted on a two-dimensional moving stage is stepped in the X or Y direction by a predetermined pitch, and a projection image of a pattern on a reticle is exposed. This operation is repeatedly performed. In this case, the wafer is finely moved in a projection optical axis direction by a Z stage on the moving stage to match the wafer surface with the optimum image plane of a projection optical system for each shot area (onto which a pattern projection image of the reticle is to be transferred), on the wafer, which is to be exposed, i.e., to perform a focusing operation. As disclosed in, e.g., U.S. Pat. No. 4,558,949, an oblique incident type focus detection system for obliquely projecting an imaging beam (non-exposure wavelength) onto the upper surface of a wafer and photoelectrically detecting reflected light is used as a focus sensor for a focusing operation. An imaging beam from this focus detection system forms a spot image or a slit image on a portion of the upper surface of a wafer normally located in substantially the center of the projection field of the projection optical system. Therefore, the positional shift amount, i.e., the focus shift amount, of the wafer surface in the optical axis direction is measured, on the basis of the photoelectrically detected signal, by referring to a position at which reflected light photoelectrically detected when the wafer surface is matched with the optimum image plane of the projection optical system is received. The position of the Z stage in the optical axis direction is servo-controlled to nullify the detected focus shift amount, thereby completing an auto focus operation.
In a stepper, such an auto focus operation is normally performed every time exposure is performed with respect to a shot on a wafer. That is, immediately after the moving stage is stepped to match the central point of one shot area with the central point of a projection image of a reticle pattern (substantially the center of the projection field), auto focus is executed by the focus detection system, and exposure is started when the Z stage comes to rest. Such a sequence is employed by the stepper.
In a stepper designed to expose a wafer, a reticle pattern to be projected generally has a rectangular shape. In contrast to this, a wafer is circular. For this reason, if shot areas are arrayed in the X and Y directions in the form of a matrix, part of a shot area near the circular peripheral portion of the wafer is omitted. The manner of omission varies depending on the arrangement of shot areas on a wafer. For this reason, after stepping is performed with respect to a shot area located in a wafer peripheral portion, the detection point (a spot image or a slit image formed by projection of oblique incident light) of the focus detection system may be located outside the wafer while the shot area is positioned in the center of the projection field.
In this case, since a focusing operation with respect to such a shot area cannot be quickly performed, exposure is performed without performing a focusing operation, or is performed while the detection point is fixed to a focus position (the position of the Z stage) in an immediately preceding shot area where a focusing operation has been properly performed. Even if, therefore, a shot area has a multi-chip structure and includes chips which can be acquired as valid chips, exposure is performed with respect to such chips without properly performing a focusing operation. This conflicts with the demand that chips including ones in a peripheral portion of a wafer should be efficiently acquired.